On the subject of why no arbosaurs, marine dinosaurs, etc.:
Well, if you accept Jeremy Rayner's views on flight evolution, the
mere existence fo birds could imply arboreal dinosaurs. Also you could say
that many of the long-armed coelurosaurs are descendants of arboreal
dinosaurs which are now ground-living (as in baboons) primarily, and
probably come up with quite a few skeletal features of the animals to
argue this. Or: maybe we haven't found them yet, or maybe we have but
haven't recognized them yet (how different are tree kangaroos from their
terrestrial relatives? Not to mention that people are still arguing over
Archaeopteryx).
As for marine forms: my guess is that there were no
constraints inherent to the body plan but that for whatever reason the
conditions weren't really right to favor it. Just a guess, but consider
that we have no marine mammals either from the Mesozoic, but as soon as we
hit the KT they go nuts and the Cenozoic sees the evolution of cetaceans,
pinnipeds, sirenians, desmostylians, and otters.Just guessing but whatever
force it was that prevented mammals from moving into the water (perhaps
this "force" was forty foot long, serpentine and with a mouth full of huge
spiky teeth, but who knows) was also the one preventing dinosaurs from
filling these niches. Although we should give Hesperornis credit. And
again, there is the possibily we haven't found them yet (a good argument
for an environment like the forest but very much less so for the water) or
that we haven't recognized them yet (an otter for example is not all that
modified it seems to me).
All in all I think this is one that may defy answer but I really
like to think it over even so...